Just got a 1989 Cherokee 4.0L with 240k. Oil pressure with a cold start is in the 40 psi range, but after it's good and warm, drops to about ~5 psi at idle in gear. Cruising at 50 with a warm engine, I'm at about 25-30 psi. The range is about the same as my '91 (20-45psi, up to 65ish when on the highway), just much further down the scale. The sending unit was replaced last month and it's had 3000 mile oil changes it's entire life, and I've got the stack of reciepts to back that up. The past two years were with Castrol GTX High Mileage 10W30 and Mobil 1 filters. No wierd crank case noises, no valve tick, nothing...nice, quiet, powerful.
So....does anyone know of anywhere that rents mechanical gauges so I can test the actual pressure? I'm wondering if the sending unit is just foobar'd. Seems anything else would have shown itself a while ago now.
FWIW, the seller (my brother) sold/gave it to me because it got tapped in the back and insurance totalled it. He did say that it's always had that low of pressure, ever since he got it, but he's taken it from Madison to Detroit and back at least 6 or 7 times in two years with no problems.
-moitz-